This invention relates to fuel combustion in boilers, water heaters, steam generators and other fuel/oxidizer mixture fired combustion devices having ultra low NO.sub.x emissions and an exceptionally high firing density. This novel device can be used to make compact, highly efficient, instantaneous water heaters, steam generators, boilers and other fuel/oxidizer mixture fired devices designed to utilize high radiant energy. In the application of this invention, a significantly smaller device can be manufactured, resulting in material and space savings and thus a lower cost.
Conventional fuel/oxidizer mixture fired water heaters, steam generators, boilers and other fired appliances, hereafter referred to collectively as appliances, typically utilize atmospheric or open flame burners. Such appliances are designed to function as convective heat transfer units due to the well known fact that the flame core has a low emissivity. The major portion of the energy released during the combustion process is transferred to the cooling agent, typically water or compressed steam, using convectional heat exchangers. In order to increase the efficiency of such systems, advanced and expensive heat exchangers, such as solid copper finned tube heat exchangers manufactured by Teledyne Laars or coils made of copper tubes with integral fins, are required. The heat transfer rate of such heat exchangers typically ranges from about 10,000 to 14,000 BTU/hr.multidot.ft.sup.2. High thermal efficiency, with low heat exchanger back pressure, is achieved by having a substantial heat transfer area which increases the overall size of the appliance. For example, the 1.8 million BTU/hr gas fired hot water boiler manufactured by Teledyne Laars, model # HH1825IN09C1A has a footprint of 98".times.28" and is 60" tall. Therefore, the specific fuel input (SFI), which is the ratio of the total fuel input (1.8 million BTU/hr) to the base area (2744 in.sup.2 or approximately 19 ft.sup.2), is about 95,000 BTU/hr.multidot.ft.sup.2. In a similar product made by the same company, model # PWO40OCN12C1CN, with a total fuel input of 400,000 BTU/hr, and a footprint of 40".times.26" (base area approximately 7.22 ft.sup.2), the SFI is about 55,400 BTU/hr.multidot.ft.sup.2. The common features of these designs are a rectangular combustion chamber 10 with a set of atmospheric burners 11 placed in the bottom as shown in FIG. 1. The front, back and both side walls of the combustion chamber are made of refractory boards 13 and a solid finned copper tube heat exchanger 14 is located on top. In this manner, the exhaust gases generated by the flame pass through the heat exchanger. Since the ratio of the radiant energy to the total energy released during the combustion process is very low, there is no reason to install additional heat exchangers on the front, back or side walls. The greatest problem with this type of combustion process is that it produces relatively high levels of NO.sub.x. These emissions exceed the South Coast Air Quality Management District (SCAQMD) requirements of 30 ppm NO.sub.x, and the Clean Air Act Amendment of 1990 (CAAA) requirements of 9 ppm NO.sub.x, as well as other NO.sub.x emission regulations.
Well known radiant burners provide combustion with a high radiant efficiency in a narrow range of calorific intensity, usually from 20,000 BTU/h.multidot.ft.sup.2 (63 kW/m.sup.2) to 100,000-200,000 BTU/ h.multidot.ft.sup.2 (315-630 kW/m.sup.2), and equivalence ratios between 0.8 and 1.2. Outside of these ranges of equivalence ratio, flames unstably lift up from the burner surface until, eventually, the entire flame lifts up, and the surface becomes non-radiant. The equivalence ratio is the ratio of oxidizer supplied for combustion to the theoretically (stoichiometrically) required amount of oxidizer for complete oxidation of the fuel.
At high thermal loadings, the range of equivalence ratios at which the burner is radiant decreases until eventually the flame lifts off the surface at all equivalence ratios. As a result of this phenomenon, the one major disadvantage of well-known radiant burners is poor turndown. Many radiant burners are able to work with a fixed fuel input, while others usually have turndowns of not more than 3:1. Other deficiencies of these burners are potential flashback problems, high pressure drop, low mechanical strength, thermal shock fragility, and high cost.
In the U.S. patent application entitled, HIGH INTENSITY, LOW NOX BURNERS, Ser. No. 08/237,306, the contents of which are herein incorporated by reference, Aleksandr S. Kushch and Mark K. Goldstein describe a burner that utilizes an advanced emissive matrix to lower the NO.sub.x emissions and increase the radiant heat output. In terms of high firing density and ultra low NO.sub.x emissions, these burners have outstanding performance. However, the lifetime of these burner systems is shortened by the degradation of the advanced emissive matrix. It is believed that this degradation is due to localized "hot spots" within the matrix. Several different materials, wire diameters and emitter thickness' have been tested with the results being the same degradation of the advanced emissive matrix.
Therefore, it is desirable to achieve a combustion process with a high calorific intensity incorporating a high ratio of radiant energy in a wide range of specific fuel inputs under excess oxidizer conditions. Further, the combustion process should not generate NO.sub.x emissions above 30 parts per million. It is also desirable to develop unique boiler/water heater designs that can utilize high radiant energy sources that will result in the savings of space and materials, increase the cost effectiveness of the unit and considerably reduce the emission of atmospheric pollutants for a range of applications from residential to utility size systems.
The inventions disclosed herein have been designed to have the above desired combustion conditions. The application of the inventions disclosed herein include, but are not limited to:
Instantaneous water heaters; PA1 Steam generators; PA1 Thermophotovoltaic generation devices; PA1 Thermophotovoltaic power appliances; and, PA1 Other fuel/oxidizer fired devices designed to utilize a high radiant energy source.